German physicist Walter H. Schottky studied under Max Planck, and conducted landmark research in vacuum electronics and semiconductor electronics. His inventions include the screen-grid vacuum tube (1915), the tetrode (a two-grid system to prevent unwanted oscillations, 1919), and the ribbon microphone and ribbon loudspeaker (1924, with Erwin Gerlach). He explained the change in electrostatics at a metal /semiconductor interface (the Schottky effect), and discovered a cellular barrier at the interface between metal and semiconductor (the Schottky barrier), which led to development of a hot carrier semiconductor diode (Schottky diode) that exhibits a nonlinear voltage/current relationship. He is also the namesake of the Schottky defect, a particular kind of point defect in a crystal lattice.